Tuesday, 30 May 2017

Hercules is famous for its two magnificent globular clusters: M13 and M92. For the rest that part of the sky is fairly empty, apart from some interesting double stars such as Ras Algethi or a few very faint and distant galaxies. And yet... in this forgotten part of the sky lies this very interesting planetary nebula. NGC6210 or more popularly the "Turtle Nebula" may be tiny but, if sky conditions permit you to increase telescope power considerably, you'll notice that it's one of the planetaries that shows the most detail. Easily visible were the two "ansae", external gas lobes that are blown out of the main shell by strong winds that come from the central star's poles. Also the extremely complex internal structures were readily visible.

The Turtle is the dying breath of a star slightly smaller than our Sun, 6,500 lightyears away, and it currently undergoes its most active phase. Clearly it's much more evolved than the proto-planetary nebula I showed you yesterday. Astronomers are dazzled about this unusual little nebula because it shows many different layers and a strange, somewhat bent overall shape. Jets of hot gas are piercing through these layers, creating complex holes and pillars. In a few thousand years the inner layers will catch up with the faint outer shell that already escaped from the unstable star just before it collapsed and the nebula will dissipate into space. Our Sun awaits the same tragic fate in about five billion years...

Monday, 29 May 2017

I've already taken you off the beaten path many times before and this time I'd like to take you waaaaay off it. Minkowsi 1-92 (or M1-92 for intimate friends) is a very interesting nebula that can be found near Albireo, the eye of Cygnus, the swan. With its 11,5 magnitude it should be accessible to most astronomy enthusiasts. The thing is that whereas most of them will indeed see it, very few will actually be able to recognise it. The reason for that is that M1-92's... incredibly tiny. I had to push my binoscope to 507x in order to see some detail because even at 285x I could've mistaken it for an ordinary star.

M1-92, aka the "Footprint Nebula", is not a planetary nebula, or not yet anyway. It's a proto-planetary, just like Frosty Leo. We're actually witnessing the death of a star here! Nuclear fusion's become critically unstable and the star collapses under its own gravity, expelling its atmosphere in the shockwave caused by the collapse. Surprisingly this doesn't happen like a balloon that's blown up. As the atmosphere around the star's equator is usually a lot thicker due to the star's rotation, the gases escape more easily around the poles and form inverted umbrella-shaped lobes. Around the equator of this particular star also lies a large dust disk that inhibits the gas from escaping even more: it looks like a waist in between the two lobes of gas. The lower lobe appears brighter because it's pointing somewhat in our direction, whereas some of the light of the upper lobe's blocked by the equatorial dust disk. In a few thousand years the tremendous heat of the dying nucleus (Currently 20,000°C on its surface that will increase to over 100,000°C as the atmosphere evaporates) will fry the expelled gas shell up to the point that it'll ionise and start to emit light. In other words, our nebula will begin to look something like this. Also in this example you can clearly see that the gas was not expelled uniformly and you can even notice some plumes of gas ("ansae") that have broken through the outer shell, blown out by the strong polar winds.

Currently our Footprint Nebula's expanding at 50km/s but given its 10,000 lightyear distance it will still take a bit of time before we'll be able to see some real change with ordinary telescopes. In the meantime, it'll remain a challenge for deep-sky fanatics. :-)

Tuesday, 23 May 2017

Some galaxies are much more active than others. Our Milky Way for instance is reasonably active but not more than that. Let's say that it's a good intermediate in every aspect. If we look into or own Local Group of galaxies, we find that the smaller Triangulum Galaxy (M33) produces four times as many stars as the Milky Way. Just look at all of those gigantic knots in its spiral arms which are all stellar nurseries up to sixty (!) times the diameter of our Orion Nebula!

But M33's just peanuts compared to some of the other galaxies out there. Take M61, for example, one of the largest members of the Virgo galaxy cluster. This is a galaxy roughly the same size as our Milky Way but twice as massive and it produces ten times as many stars. Sometimes starburst activity's caused by the close interaction or even collision with other galaxies, as is the case with M82 for instance. Near M61 we find a couple of smaller companion galaxies, one of which (NGC4301) was easily visible in the same field of view. These companions, however, are certainly not massive enough to trigger such freak activity in its much larger parent and so we have to look for an additional explanation in this case. This explanation's found right in the galaxy's core: a supermassive black hole. Actually, it's not that difficult to see a black hole. As I explained about M77, black holes are indeed "black" because not even light can escape from it. Yet they emit a lot of energy, the result of collisions between the atoms that are squeezed together around the black hole just before they fall into it; the so-called Hawking radiation. Therefore, when you see a galaxy with an unusually bright core that looks almost stellar, you can bet on it that it contains a gigantic black hole.

Another interesting fact about M61 is that its spiral arms are not really curved but more angular-shaped, hence its nickname: Hexagon Galaxy. Over the last century no less than six supernovae have been observed in this galaxy and this puts it on a divided second place with M83. The absolute king of supernovae remains the "Fireworks Galaxy" (NGC6946) in Cygnus with ten so far.

Friday, 19 May 2017

Not really knowing what he was so upset about I put on a confused face and made the associated Italian hand gesture. After seven years in this beautiful country I've become quite integrated. :-)

"You're always ranting", he went on to clarify, "that street lights attract burglars rather than scare them off and blablabla but it's obvious that you're wrong!"

After he'd calmed down he finally explained what had happened. Friends of his had got burgled last Thursday, for the third time. Apparently they live in an isolated house in a street without any lights. According to him it was obvious that the absence of lights makes the house very attractive to burglars and the local authorities should do something about it immediately. Yet I shook my head in disagreement. A nice, isolated house with an easy escape route will always be on top of the burglars' list, I argued, but the absence of lights has nothing to do with it.

"Actually, the house wasn't immersed in darkness at all", I said, "so this proves my point that burglars prefer illuminated houses over those in the total dark."

My father-in-law, convinced that I was boasting, didn't take well to my refusal to accept his wisdom.

"How do you know that? You don't even know where they live!"

"True, but I know for certain that the house was bathing in light. We could walk over there at night, even try to force a door or window and we wouldn't need to use a torch at all."

"Oh really? You're just mouthing off! You don't know them. You don't know their house. You don't know the area where they live. I tell you that it's completely dark down there! It'd look better on you if you admitted that you're wrong."

"I don't need to know where they live to know for certain that the house was brightly illuminated: there was a full Moon."

It turned out that also the other two times when the house got burgled the Moon was shining brightly in the sky. As you all know, a full Moon illuminates more than all street lights combined. Therefore my point of view's still standing and has actually become stronger since in this case the burglars have always chosen nights with as much light as possible. Street lights make burglars invisible, rather than visible. They can move about without having to use a torch and there are enough shadows to hide in. Yes, it is possible that you see them but even if you did, would they draw your attention? Probably not.

In a completely dark environment, however, burglars are compelled to use a torch which, against a pitch-black background, really draws the attention. Also anti-burglary lights are much more efficient in the total dark. First of all because they can blind a burglar - who had adapted to total darkness - for minutes, and secondly because such a light would draw the attention from a mile away, whereas in an already illuminated street it wouldn't.

Monday, 15 May 2017

Looking at the night's sky is a dazzling experience, especially if you're fortunate enough to have a sufficiently dark sky. The sight of those millions of stars can't possibly leave anyone indifferent and makes most people feel tiny and humble. What only few people realise, however, is that every single one of those stars is a unique world on its own. Astronomers are already astonished by the incredbile diversity of the eight planets of our Solar System and its hundreds to even thousands of moons and dwarf planets. From the furnace of Venus where it's raining pure sulphuric acid, to the water geysers on Enceladus... from the strange methane eruptions on Mars to the all-destroying storms of Neptune that blow faster than the speed of sound... from the violent solar prominences that extend over ten times the Earth's diameter to Titan, the only moon in our solar system to hide its surface under a thick layer of clouds. Even tiny Pluto turned out to be a much more spectacular world than we had anticipated at first. If we already find so many places in our own Solar System, so weird that not even a Sci-Fi writer on dope could make them up, imagine what else must lie hidden up there, so many lightyears away.

But often even distant stars may reveal some very unusual features even to small amateur telescopes. Alpha Herculis for example, also known by the proper name Ras Algethi, is the fourth brightest star in the constellation representing the famous Greek hero. It doesn't look particularly bright, nor insteresting from our Earthly point of view. Until... you take a telescope and point at it. Then you'll notice that this ordinary, orangy star hides a little surprise. You'll need to look carefully because it's close to the main star, but you'll discover that Ras Algethi has a little companion. Well... little... we have to put things into perspective here.

Ras Algethi is a red giant. It's so big that if it were at the heart of our Solar System, it would extend beyond the orbit of Mars! Its companion is a double star on its own, but the two components are so close that they're impossible to separate with ordinary telescopes. Both components are also much bigger than our Sun and have over two solar masses each. Officially they're yellow but I rather saw the companion as greenish. As close as they seem through the telescope, in reality Ras Algethi and its companions are 550 times the distance between the Earth and the Sun apart. But even from this great distance, the double companion would shine much more brightly in our sky than a full Moon.

And this... is just one example of one of the less-fashionable stars in our sky...

Friday, 12 May 2017

The universe is all but a static place. As inconceivably vast as the distances between celestial objects may seem to us tiny humans, what are a couple of million lightyears when you've got billions of years of time? Even the great galaxies, entities as large as hundreds of thousands of lightyears across and containing billions to even trillions of stars may face calamities similar to a meteorite hitting the Earth. I've already talked about interacting galaxies many times before. Sometimes they're zooming past each other like Formula One cars going through a bend, such as this example. Sometimes they're merely bound by gravity such as the enormous Virgo Supercluster. In extreme cases they may even crash into each other, merge or be left completely deformed. Also the Andromeda Galaxy will eventually crash into our Milky Way.

But there are also slightly less dramatic situations in which two galaxies simply hover close by and slowly devour each other. This seems to be the case with the two galaxies on my sketch: NGC4435 (right) and NGC4438 (left), which are currently only 100,000 lightyears apart. In comparison, Andromeda is 2.5 million lightyears away from us. Well, hovering is not exactly the correct term. They have come very close to one another many millions of years ago and have literally ripped each other apart in the process, but certainly not at the same speed as NGC672 and IC1727 in the example I linked to above. They're more in a sort of orbit at the heart of the Virgo Cluster, in an area we call "Markarian's Chain". This is a large chain of bright galaxies within this cluster, which is already a stunning sight in binoculars.

NGC4435 seems to be completely stripped of its spiral arms, whereas the somewhat larger NGC4438's still trying to hold on to most of its matter. Its nucleus is highly distorted and in stead of spiral arms it shows long, irregular trails of stars and gas. Given the relatively slow orbit speed and consequently limited tidal pull between the two, scientists have been looking for other explanations why these two are in such a bad way. The most likely answer is M86, a gigantic lenticular galaxy not far away. Recently, filaments of ionised gas have been discovered between this giant (much like M87) and NGC4438. It's not only M86's size that's squeezing the two little ones, but also its incredible speed. M86's one of the few galaxies in the supercluster that's headed towards us in stead of away from us and it's doing it at the respectable speed of 244km/s. That doesn't seem like much as such but since most of the galaxies in the cluster are flying in the opposite direction with speeds well over 1,000km/s the movement of M86 becomes much more significant. As big as it is, M86's being stretched out by ram pressure. Imagine what's happening to your hair when you're doing 100mph in a convertible car. Now imagine that you're standing by the side of the road and a fifty-tonne truck zooms past at 100mph. That's what NGC4435 and NGC4438 are experiencing right now!

In popular culture, these two galaxies are referred to as the "Eyes Galaxies" for obvious reasons. Since there's not much of them left apart from their nuclei containing mostly older, yellow-orangy stars, these eyes are basically... yellow.

Monday, 8 May 2017

Already as a kid I was seriously obsessed by the night's sky. I remember one night in early December 1984... It was just past three and my parents were fast asleep. Swiftly I got out of bed and began to raise the shutter of the terrace door, which was a tricky thing to do because it squeaked a lot. It wouldn't be the first time that my father came running to my room to give me a good lecture about sleeping and school tomorrow. But this time I didn't hear anyone running down the corridor. There was just my little brother, my loyal partner in crime, who had also woken up and lent me a hand. Right... the shutter was up and the door towards our large terrace was open. I grabbed my 60mm Vixen refractor and manoeuvred it very cautiously through the door. It was too much of a hassle to disassemble it with its equatorial mount so I had to be careful not to hit the door posts with one of the opened tripod's legs. This time I wouldn't be observing from the terrace... risky because too close to my parents' bedroom... so I carried the telescope down the stairs and towards the back of our garden. When I looked up, I saw a sky I hadn't seen before. Orion was about to disappear behind the trees in the west and it was now Leo that ruled the south with the vast and strange constellation of Virgo following closely behind. This was all new to me because usually as from April, when these constellations resplend in the evening, it gets dark late and as such I hadn't had the opportunity to see them yet (school tomorrow... remember?). However, I knew that there was a very interesting object east of Virgo, in a dark corner of the sky that belonged to Serpens Caput, the snake's head: M5. I turned my telescope in the right direction and saw a bright, somewhat grainy patch. My telescope was too small to resolve it into individual stars, but already then I noticed its somewhat irregular shape.

Now, thirty-three years later, M5's still my favourite globular cluster. According to recent estimates it may even contain up to 500.000 stars (!), many of which will easily resolve in a modest telescope. This makes it one of the largest globular clusters known and it is also one of the oldest with an estimated age of 13 billion years. That's about as old as our Universe! At a distance of 24.500 lightyears, it spans some 200 lightyears across. Can you imagine that? Hundreds of thousands of stars in such a confined space! This explains why many stars in globular clusters, notwithstanding the fact that they're among the oldest stars in our Universe and should therefore be cool, red giants, still appear very hot and blue. The extreme proximity of neighbouring stars can literally blow away the stars' atmospheres, exposing their much hotter interior!

But who cares about all of this theory? Just look at this incredible spectacle and probably you'll understand why I wanted so desperately to sneak out in the middle of the night in order to get a glimpse of it.

Tuesday, 2 May 2017

The Virgo supercluster is a gigantic accumulation of galaxies that not only contains the large, central Virgo-Coma Berenices cluster but also about a hundred other galaxy groups. Also the group to which our own Milky Way belongs, together with e.g. Andromeda and Triangulum, is gravitationally bound to the Virgo supercluster. Its centre lies approximately 65 million lightyears away from us and exactly there we find this enormous galaxy, called M87. It belongs to the older group of elliptical galaxies and as such doesn't display any spiral arms anymore. Actually, it's not even a disk but nearly a perfect sphere, the outer halo of which extends up to a diameter of half a billion lightyears! In comparison, our Milky Way's only 100 million lightyears across. Where our galaxy's accompanied by about 200 globular clusters, there are at least 12.000 orbiting M87! Its mass is estimated at 2 to 3 trillion solar masses!

Okay... are you still with me? Good. At first sight this magnificent galaxy's not the most interesting for us amateur astronomers because it just looks like a fuzzy patch. No spectacular spiral arms or structures... nothing at all. But as so often, it's the tiny detail that makes all the difference. It doesn't come as a surprise that such a colossal galaxy houses a supermassive black hole in its core. Virgo A is indeed one of the biggest black holes in our part of the known universe - scientists believe it's as big as our solar system, imagine that! - and also one of the most powerful radio and x-ray sources in the sky. Now it so happens that this black hole's responsible for another, strange phenomenon: a huge jet of matter, 5.000 lightyears long, that's being ejected from the galaxy's core. There's actually a second jet on the opposite side, but that one's much more difficult to see and remains invisible to amateur instruments. What I wanted to see, however, was that main jet, so I pushed my binoscope to 507x and concentrated as well as I could. And there it was... I have to say that it was extremely difficult to see using averted vision; the halo around the galaxy's nucleus is so bright that it tends to hide the jet. And yet, there was definitely some sort of streak popping out towards the bottom-left of the core.

Also interesting to note on the sketch are two very faint and much more distant galaxies near the
top-left corner of the field of view: PGC139919
(300 million lightyears away) and PGC41342 (1,3 billion lightyears away) by the edge. Obviously they have nothing to do with giant M87 or with each other and are merely a line-of-sight coincidence.